The present invention relates to a hydraulic drive system for a construction machine including a swing control system, such as a hydraulic excavator. More particularly, the present invention relates to a hydraulic drive system wherein, when a hydraulic fluid from a hydraulic pump is supplied to a plurality of actuators, including a swing motor, through respective associated directional control valves, a delivery rate of the hydraulic pump is controlled by a load sensing system and differential pressures across the directional control valves are controlled by respective associated pressure compensating valves.
JP, A, 60-11706 discloses a hydraulic drive system for controlling a delivery rate of a hydraulic pump by a load sensing system (hereinafter referred to also as an LS system). Also, JP, A, 10-37907 discloses a hydraulic drive system for a construction machine including a swing control system, the hydraulic drive system including an LS system and being intended to realize independence and operability of the swing control system. A 3-pump system mounted on an actual machine is also disclosed as an open-center hydraulic drive system for a construction machine including a swing control system, the hydraulic drive system being intended to realize independence of the swing control system. Further, JP, A, 10-89304 discloses a hydraulic drive system wherein a delivery rate of a hydraulic pump is controlled by an LS system and a pressure compensating valve is given a load dependent characteristic.
In the hydraulic drive system disclosed in JP, A, 60-11706, a plurality of pressure compensating valves each include means for setting, as a target compensation differential pressure, a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators. In the combined operation where a plurality of actuators are simultaneously driven, there occurs a saturation state that the delivery rate of the hydraulic pump is not enough to supply flow rates demanded by a plurality of directional control valves. In such a saturation state, the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure is lowered, and correspondingly the target compensation differential pressure of each pressure compensating valve is reduced. As a result, the delivery rate of the hydraulic pump can be redistributed in accordance with a ratio between the respective flow rates demanded by the actuators.
In the hydraulic drive system disclosed in JP, A, 10-37907 and the 3-pump system mounted on an actual machine, an independent open-center circuit using an independent hydraulic pump is constructed for a swing section, which includes a swing motor, separately from a circuit for the other actuators, whereby independence and operability of the swing control system is ensured.
In the hydraulic drive system disclosed in JP, A, 10-89304, a plurality of pressure compensating valves each have hydraulic pressure chambers constructed as follows. A pressure bearing area of a hydraulic pressure chamber, to which an input side pressure of a directional control valve is introduced and which produces a force acting in the valve-closing direction, is set to be greater than a pressure bearing area of a hydraulic pressure chamber, to which an output side pressure of the directional control valve is introduced and which produces a force acting in the valve-opening direction. The pressure compensating valve is thereby given such a load dependent characteristic that, as a load pressure of each associated actuator rises, the target compensation differential pressure of the pressure compensating valve is reduced (i.e., the pressure compensating valve is throttled) to decrease a supply flow rate to the actuator. As a result, the actuators on both the lower and higher load sides can be operated with good operability in a stable manner without hunting.
The conventional hydraulic drive systems described above however have the following problems with the swing control system.
JP, A, 60-11706: problems {circle around (1)} and {circle around (2)}
JP, A, 10-89304: problems {circle around (2)} and {circle around (3)}
JP, A, 10-37907: problem {circle around (4)}
Open-center 3-pump system mounted on actual machine: problem {circle around (4)}
{circle around (1)} jerky feel in operation at start-up of swing alone
{circle around (2)} change of the swing speed at shift from operation of swing alone to combined operation including swing and vice versa.
{circle around (3)} extreme drop of the swing speed at start-up of combined operation including swing
{circle around (4)} increase in cost and space and complicated circuit configuration due to provision of a separate circuit
(1) JP, A, 60-11706
When the hydraulic drive system including the LS system, disclosed in JP, A, 60-11706, is applied to the swing control system, it is difficult to keep balance between load sensing control (hereinafter referred to also as LS control) of the hydraulic pump and a flow rate compensating function of the pressure compensating valve due to an inertial load of the swing control system. This is because a difficulty occurs in keeping balance between response of the pressure compensating valve and response in the LS control of the hydraulic pump due to the following reasons when a swing driving pressure is controlled in a stage of shift from swing acceleration to steady rotation.
(1) In a swing start-up and acceleration mode, the pump LS control is performed so as to raise a delivery pressure of the hydraulic pump depending on the swing start-up pressure for holding a constant flow rate.
(2) To hold constant a differential pressure across a throttling element of the directional control valve, the pressure compensating valve is operated in a direction to increase a flow rate passing itself that tends to reduce upon a rise of the load pressure.
(3) When the swing reaches a steady speed, the swing driving pressure is lowered and therefore the pump LS control is not required to control the delivery pressure of the hydraulic pump so high as in the swing start-up and acceleration mode. Hence the pump LS control is performed in a direction to lower the delivery pressure of the hydraulic pump.
(4) Upon a lowering of the swing driving pressure, the pressure compensating valve is operated in a direction to reduce the flow rate passing itself that tends to increase.
Because of quick shift from (1) to (4), the swing operation becomes jerky (above problem {circle around (1)}).
In the combined operation, as described above, there occurs a saturation state that the delivery rate of the hydraulic pump is not enough to supply flow rates demanded by a plurality of directional control valves. Corresponding to such a saturation state, the target compensation differential pressure of each pressure compensating valve is reduced, and the delivery rate of the hydraulic pump is redistributed in accordance with a ratio between the respective flow rates demanded by the actuators. With that function, even in the combined operation, the actuators are operated, although slightly slowed down, by the hydraulic fluid distributed at the ratio depending on the intended operations, whereby a feel in the operation is not impaired.
However, such slowdown likewise occurs in the swing operation, and during the combined operation including swing, the swing speed is also reduced as with one or more other actuators. This slowdown gives rise to change of the swing speed at shift from the swing-combined operation to the swing-alone operation and vice versa, thus causing the operator to feel awkward (above problem {circle around (2)}).
(2) JP, A, 10-89304
In the hydraulic drive system disclosed in JP, A, 10-89304, since the pressure compensating valve is given a load dependent characteristic, the target compensation differential pressure of the pressure compensating valve is reduced in response to a rise of the load pressure of the swing motor at the start-up of swing alone, and when the swing motor shifts to a steady sate, the target compensation differential pressure of the pressure compensating valve is also returned to the original value in response to a lowering of the load pressure of the swing motor. As a result, the swing can be started up without causing a jerky feel in operation. However, when the delivery rate of the hydraulic pump comes into a saturation state in the combined operation, the delivery rate of the hydraulic pump is redistributed in accordance with a ratio between the respective flow rates demanded by the directional control valves, as with the hydraulic drive system disclosed in JP, A, 60-11706. Accordingly, the swing speed is changed at shift from the swing-combined operation to the swing-alone operation and vice versa, thus causing the operator to feel awkward (above problem {circle around (2)}).
Further, since the pressure compensating valve is given a load dependent characteristic, the target compensation differential pressure of the pressure compensating valve for the swing section is reduced depending on the condition of the delivery rate of the hydraulic pump at the start-up of the swing-combined operation. In addition, the target compensation differential pressure is also reduced due to the load dependent characteristic as the load pressure of the swing motor rises up to a relief pressure. Such a reduction in the target compensation differential pressure continues until the swing motor shifts to the steady sate. As a result, the swing speed is extremely lowered as compared with the speeds of other actuators at the start-up of the swing-combined operation, whereby swing operability at the start-up of the swing-combined operation is deteriorated (above problem {circle around (3)}).
(3) Hydraulic Drive System Disclosed in JP, A, 10-37907 and Open-center 3-Pump System Mounted on Actual Machine
In the hydraulic drive system disclosed in JP, A, 10-37907, the swing control system is constructed by a separate open-center circuit to ensure satisfactory swing operability in the LS system. Also, in the open-center 3-pump system mounted on an actual machine, the swing control system is constructed as a separate open-center circuit to ensure satisfactory swing operability.
More specifically, in the open-center system, when the driving pressure rises at the swing start-up, a flow rate of the hydraulic fluid returning to a reservoir through a center bypass fluid line is increased, which reduces a flow rate of the hydraulic fluid passing a throttle of the directional control valve for the swing section. A flow rate of the hydraulic fluid supplied to the swing motor is therefore restricted in the swing start-up and acceleration mode. When the swing speed reaches a steady speed, no restriction is imposed on the supply flow rate to the swing motor because of the driving pressure being not so high as at the swing start-up, and the hydraulic fluid is supplied to the swing motor at a flow rate corresponding to an opening of the throttle of the directional control valve for the swing section. The swing can be thereby smoothly started up without causing a jerky feel in operation for starting up the swing solely unlike the LS control.
Although the above problem {circle around (2)} occurs in not only the LS system but also the open-center system, change of the swing speed is not caused in the hydraulic drive system and the open-center 3-pump system mounted on an actual machine, which are disclosed in JP, A, 10-37907, because the swing control system is constructed as the separate open-center circuit and independence of the swing control system is realized.
However, in the hydraulic drive system disclosed in JP, A, 10-37907 and the 3-pump system mounted on an actual machine, the swing control system must be constructed as a separate circuit in parallel to the system for the other actuators. Correspondingly, a cost is pushed up and a space required for installation is increased. In addition, a hydraulic pump for the swing control system must be separately provided. In the system disclosed in JP, A, 10-37907, particularly, a signal line is required to keep power balance between the swing control system and the LS system which are arranged in parallel, and hence the circuit configuration is complicated (problem {circle around (4)}).
An object of the present invention is to provide a hydraulic drive system including a swing control system, which enables swing operation to be accelerated for shift to a steady state without causing a jerky feel at the start-up of swing alone and combined operation including swing, which can suppress change of the swing speed at shift from the swing-alone operation to the swing-combined operation and vice versa, which can avoid the swing speed from extremely reducing as compared with the speeds of one or more other actuators at the start-up of the swing-combined operation, thereby ensuring superior swing operability and swing independence, and which is free from problems resulted from providing a separate circuit, such as an increase in cost and space and complication of the circuit configuration.
(1) To achieve the above object, the present invention provides a hydraulic drive system comprising a hydraulic pump, a plurality of actuators, including a swing motor, which are driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, a plurality of pressure compensating valves for controlling respective differential pressures across the plurality of directional control valves, and pump control means for load sensing control to control a pump delivery rate such that a delivery pressure of the hydraulic pump is held a predetermined value higher than a maximum load pressure among the plurality of actuators, wherein the hydraulic drive system further comprises first means provided respectively in those of the plurality of pressure compensating valves, which are not for a swing section associated with the swing motor, and setting, as a target compensation differential pressure, a differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators; second means provided in the pressure compensating valve for the swing section and setting a target compensation differential pressure of that pressure compensating valve; third means provided in at least one of the plurality of pressure compensating valves, which is for the swing section, and reducing the target compensation differential pressure set by the second means when a load pressure of the swing motor rises, thereby giving a load dependent characteristic to the pressure compensating valve for the swing section; and fourth means provided in the pressure compensating valve for the swing section and setting a lower limit of the target compensation differential pressure that is set by the second means and modified by the third means.
With the present invention thus constructed, since the third means is provided in the pressure compensating valve for the swing section to give it the load dependent characteristic, the pressure compensating valve for the swing section finely adjusts the flow rate passing the same depending on change in the load pressure of the swing motor at the swing start-up, whereby the swing motor is smoothly accelerated and shifted to the steady state.
Also, the second means for setting the target compensation differential pressure of the pressure compensating valve for the swing section may be means for setting, as the target compensation differential pressure, the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators as with the first means. In this case, by providing the fourth means as set forth above, the fourth means functions as lower limit setting means for limiting both reduction in the target compensation differential pressure itself set by the second means and reduction in the target compensation differential pressure due to the load dependent characteristic given by the third means (see (2) below). With this function, when the target compensation differential pressure of the pressure compensating valve for the swing section is going to reduce upon the delivery rate of the hydraulic pump coming into the saturation state, or when the target compensation differential pressure of the pressure compensating valve for the swing section is going to reduce in accordance with the load dependent characteristic upon a rise of the load pressure of the hydraulic pump, or when both of the above phenomena occur at the same time, the fourth means limits the reduction of the target compensation differential pressure so that the hydraulic fluid is supplied to the swing motor with priority. As a result, change of the swing speed is suppressed at shift from the swing-alone operation to the swing-combined operation, and vice versa. Further, at the start-up of the swing-combined operation, the swing speed is prevented from being extremely slowed down as compared with the speed of another actuator, whereby superior swing operability and swing independence can be ensured.
The second means for setting the target compensation differential pressure of the pressure compensating valve for the swing section may be means for setting, as the target compensation differential pressure, a value not changed depending on the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators. In this case, the fourth means functions as lower limit setting means for limiting reduction in the target compensation differential pressure due to the load dependent characteristic given by the third means (see (3) below). With this function, even when the delivery rate of the hydraulic pump comes into the saturation state, the target compensation differential pressure of the pressure compensating valve for the swing section is not reduced. Also, when the target compensation differential pressure of the pressure compensating valve for the swing section is going to reduce in accordance with the load dependent characteristic upon a rise of the load pressure of the hydraulic pump, the fourth means limits the reduction in the target compensation differential pressure. Thus, even when the reductions in the target compensation differential pressure due to the saturation and the load dependent characteristic occur solely or simultaneously, the hydraulic fluid is supplied to the swing motor with priority. As a result, change of the swing speed is suppressed at shift from the swing-alone operation to the swing-combined operation, and vice versa. Further, at the start-up of the swing-combined operation, the swing speed is prevented from being extremely slowed down as compared with the speed of another actuator, whereby superior swing operability and swing independence can be ensured.
Additionally, since the above-described functions are achieved without providing a separate circuit, such problems as an increase in cost and space and complication of the circuit configuration are avoided.
(2) In the above (1), preferably, the second means is means for setting, as the target compensation differential pressure, the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators as with the first means, and the fourth means functions as lower limit setting means for limiting both reduction in the target compensation differential pressure itself set by the second means and reduction in the target compensation differential pressure due to the load dependent characteristic given by the third means.
With that feature, as set forth in the above (1), when the target compensation differential pressure of the pressure compensating valve for the swing section is going to reduce upon the delivery rate of the hydraulic pump coming into the saturation state, or when the target compensation differential pressure of the pressure compensating valve for the swing section is going to reduce in accordance with the load dependent characteristic upon a rise of the load pressure of the hydraulic pump, or when both of the above phenomena occur at the same time, the fourth means limits the reduction of the target compensation differential pressure so that the hydraulic fluid is supplied to the swing motor with priority, whereby superior swing operability and swing independence can be ensured.
(3) In the above (1), the second means may be means for setting, as the target compensation differential pressure, a value not changed depending on the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators. In this case, the fourth means functions as lower limit setting means for limiting the reduction in the target compensation differential pressure due to the load dependent characteristic given by the third means.
With that feature, as set forth in the above (1), even when the delivery rate of the hydraulic pump comes into the saturation state, the target compensation differential pressure of the pressure compensating valve for the swing section is not reduced. Also, when the target compensation differential pressure of the pressure compensating valve for the swing section is going to reduce in accordance with the load dependent characteristic upon a rise of the load pressure of the hydraulic pump, the fourth means limits the reduction in the target compensation differential pressure. Thus, even when the reductions in the target compensation differential pressure due to the saturation and the load dependent characteristic occur solely or simultaneously, the hydraulic fluid is supplied to the swing motor with priority, whereby superior swing operability and swing independence can be ensured.
(4) In the above (1)-(3), preferably, the fourth means is biasing means for applying a biasing force to a spool of the pressure compensating valve for the swing section in the valve-opening direction when the target compensation differential pressure set by the second means and modified by the third means reaches a predetermined value.
With that feature, the fourth means prevents the target compensation differential pressure of the pressure compensating valve for the swing section from reducing down below a value corresponding to the biasing force applied by the biasing means.
(5) In the above (4), preferably, the biasing means is a lower limit setting spring acting on the spool of the pressure compensating valve for the swing section and biasing the spool in the valve-opening direction when the target compensation differential pressure set by the second means and modified by the third means reaches the predetermined value.
With that feature, the biasing means applies the biasing force to the spool of the pressure compensating valve for the swing section in the valve-opening direction when the target compensation differential pressure of the pressure compensating valve for the swing section reaches the predetermined value.
(6) In the above (1) and (2), preferably, the fourth means is biasing means for always adding a supplement value to the target compensation differential pressure that is set by the second means and modified by the third means, and the directional control valve for the swing section is constructed such that meter-in variable throttles thereof each have an opening area smaller than that in the directional control valves not for the swing section by an amount of the target compensation differential pressure corresponding to the supplement value added by the biasing means.
With that feature, the fourth means restricts the reduction in the target compensation differential pressure of the pressure compensating valve for the swing section by an amount corresponding to the supplement value added by the biasing means, thereby setting a lower limit of the target compensation differential pressure.
(7) In the above (6), preferably, the biasing means is a swing priority spring always acting on the spool of the pressure compensating valve for the swing section in the valve-opening direction.
With that feature, the fourth means always adds the supplement value to the target compensation differential pressure of the pressure compensating valve for the swing section.